1. Field of the Invention
The present invention relates to a fine-particle measuring apparatus for effectinq measurement of fine particles (foreign matter) present in a process unit for formation of films, etching, cleaning, etc., for example, to a contamination inspection apparatus for detecting foreign matter on a wafer.
2. Description of the Prior Art
FIG. 3 shows the arrangement of a conventional fine particle measuring apparatus, as being a first prior art apparatus, disclosed, for example, in Japanese Patent Public Disclosure No. 63-30570 (1988). The apparatus is designed to measure fine particles attached to the surface of a wafer. In the figure, the reference numeral 1 denotes a substrate (wafer) for a semiconductor device which is an object of measurement, 2 a fine particle, 3 a laser light source (a light source for generating parallel rays), 4 a polarizer, 5 objective lenses, 6 a photodetector which converts light into an electrical signal, and 7 an electronic circuit device which processes information output from the photodetector 6 to obtain the results of measurement of fine particles. The reference numeral 8 denotes a driving mechanism for moving the position of the wafer 1.
The operation of the first prior art apparatus will next be explained. Laser light emitted from the laser light source 3 is applied almost parallel to the surface of the wafer 1 As the laser light that is applied to the wafer surface, for example, S-polarized laser light is employed. The S-polarized laser light is scattered by the fine particle 2. However, since the surface of the fine particle 2 has minute irregularities, the scattered light contains a large amount of P-polarized light. On the other hand, the medium constituting the measuring atmosphere is usually a gas, i.e., air, and light that is scattered by gas molecules in the manner of Rayleigh scattering contains no P-polarized light component. Accordingly, the scattered light caused by gas molecules is cut off by the polarizer 4 disposed in such a manner as to intercept the S-polarized light component. As a result, only the P-polarized light component of the scattered light from the fine particle 2 is received by the photodetector 6 through the objective lenses 5, and the result of measurement is obtained in the electronic circuit device 7. The driving mechanism 8 is provided to measure the distribution of fine particles on the wafer surface.
FIG. 4 is a sectional view showing the arrangement of another conventional fine-particle measuring apparatus, as being a second prior art apparatus, disclosed, for example, in A. Shintani et al.: J. Electrochem. Soc. 124, No. 11 (1977), p. 1771. In the figure, the reference numeral 3 denotes a laser light source, 9 an observation zone which is spatially limited by a light-receiving lens system 10 and which contains fine particles which are to be measured, 6 a photodetector, and 11 and optical trap for minimizing stray light in the measuring apparatus. In actual use, this prior art apparatus is connected to a process unit by the use of a capillary (tube) adapted to suck in a gas containing fine particles dispersed in the process unit, thereby indirectly measuring the fine particles in the process unit.
FIGS. 5(a) and 5(b) are plan and front views showing the operating principle of an in-situ particle flux monitor, as a third prior art apparatus.
Laser light from a laser 3 is reflected a large number of times between a pair of mirrors 21 disposed parallel to each other, thereby enlarging the two dimensional observation zone. When fine particles 2 are passing through the zone, light is scattered thereby and this scattered light is received by a photodetector 6 to thereby measure fine particles. It should be noted that the reference numeral 22 denotes reflecting condensers, while the numeral 23 denotes a beam stopper. This apparatus is used within a process unit.
The above-described prior arts suffer, however, from the following problems. In the fine-particle measuring apparatus according to the first prior art apparatus, it is necessary for a fine particle under measurement to have minute irregularities on its surface which are not much smaller in size than the wavelength of the laser light and, thus, it is difficult with this prior art apparatus to measure fine particles which have relatively smooth surfaces and fine particles which have relatively small particle diameters. These fine particles may be measured by the use of P-polarized laser light or non-polarized laser light in place of S-polarized laser light. In such a case, however, the Rayleigh-scattered light (P-polarized light) scattered by the gas that constitutes the measuring atmosphere cannot be cut off with the polarizer 4 and the S/N ratio cannot therefore be increased. Thus, it is difficult with the prior art apparatus to measure fine particles having small diameters. In addition, since this prior art apparatus is not adapted to measure fine particles in a process unit but is designed for off-line inspection, it is difficult to apply the prior art apparatus to the measurement of fine particles in a process unit even if the polarizer 4 and the objective lenses 5 (constituting in combination a microscope) are disposed in close proximity to the wafer 1 to limit the observation zone.
The fine-particle measuring apparatus according to the second prior art apparatus involves the problem that it is impossible to measure fine particles on the surface of a wafer set in the process unit and, with regard to the fine particles suspended in the process unit, it is only possible to measure those which can be successfully sucked and transported into the measuring apparatus.
The fine-particle measuring apparatus according to the third prior art apparatus is capable of measuring suspended fine particles but cannot measure those adhered to the wafer surface. In addition, since the optical system (comprising the laser light source 3, the mirrors 21, the photodetector 6, etc) is installed inside a process unit, in the case, for example, of a film forming process by atomspheric pressure thermal CVD, it is difficult to measure fine particles on the surface of a wafer heated to a high temperature or those which are suspended above the wafer surface during a film forming process. Even when no film is being formed, the presence of the apparatus also causes substantial changes in environment (e.g., the gas flow, temperature distribution, etc.) in the vicinity of the wafer. In an etching or cleaning process also, it is difficult to effect measurement without causing critical disturbances. In addition, since the measuring system according to this prior art apparatus has no means for eliminating signals (i.e., background noise) caused by light that is scattered by gas molecules constituting the measuring atmosphere medium in the manner of Rayleigh scattering, it is difficult to measure fine particles having such small particle diameters that the intensity of light scattered thereby is too weak to ensure that the desired signal will not be obscured by background noise.